Generally, a silicon nitride sintered body is produced by adding a sintering auxiliary selected from MgO, CaO, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, Yb.sub.2 O.sub.3, HfO.sub.2, Sc.sub.2 O.sub.3, Ce.sub.2, ZrO.sub.2, SiO.sub.2, Cr.sub.2 O.sub.3 and AlN to an alpha-type or beta-type silicon nitride powder, forming the obtained mixed powder and sintering it at 1600 to 2100.degree. C. under a nitrogen pressure of 1 to 100 atmospheres. Further, the silicon nitride sintered body is produced by sealing such a mixed powder in a glass capsule and sintering it by HIP (hot isostatic pressing) under a gas pressure of about 1000 atmospheres, or sintering it by HP (hot pressing) under a pressure of about 20 to 40 MPa at 1600 to 1800.degree. C. under a nitrogen pressure of 1 atmosphere.
However, the thermal conductivity of the silicon nitride sintered bodies obtained according to these prior arts is not so high of 10 to 70 W/mK.
Hereunder, the thermal conductivities of silicon nitride sintered bodies obtained according to the prior arts are exemplified.
Example 1) Am. Ceram. Soc. Bull., Vol. 57 No. 12, pp. 119-1122 (1978)
The thermal conductivity of a sintered body obtained by adding 15.4 mol % of MgO to an alpha-silicon nitride ponder and sintering it by hot pressing at 1750.degree. C. for 30 minutes under a pressure of 20 MPa is 55 W/mK.
The thermal conductivity of a sintered body obtained by adding 31 mol % of Al.sub.2 O.sub.3 to an alpha-silicon nitride powder and sintering it by hot pressing at 1750.degree. C. for 30 minutes under a pressure of 20 MPa is 10 W/mK.
Example 2) J. Ceram. Soc. Jpn., 92 2!, pp. 174-81 (1989)
The thermal conductivity of a sintered body obtained by adding 6 mol % of Y.sub.2 O.sub.3 to an alpha-silicon nitride powder and sintering it by HIP at 1850.degree. C. for one hour under a pressure of 60 MPa is 70 W/mK.
Example 3) J. Mater. Sci., 3!, pp 915-16 (1984)
The thermal conductivity of a sintered body obtained by adding 5 weight % of MgO to an alpha-silicon nitride powder and sintering it by hot pressing at 1700.degree. C. for one hour under a pressure of 14.7 MPa is 56 W/mK.
Example 4) Test Example by the Present Inventors
The thermal conductivity of a sintered body obtained by adding 5 weight % of Y.sub.2 O.sub.3 and 2 % weight of Al.sub.2 O.sub.3 to an alpha-silicon nitride powder, forming the mixture by a mold, further forming it by CIP under a pressure of 5 tons/cm.sup.2 and sintering it at 1850.degree. C. for 6 hours under the nitrogen pleasure of 9 atmospheres is 25 W/mK.
As described above, all silicon nitride sintered bodies obtained according to the prior arts have low thermal conductivity and leave the door open further improvements, and hence the development of a new sintered body having high thermal conductivity has been demanded fairly in the field concerned.
In addition, in the case of metal materials to be used widely (copper, stainless steel, etc.), it is impossible to use them without performing cooling, since strength, oxidation-proof property and corrosion resistance of them are reduced remarkably under the condition beyond 500.degree. C.; on the other hand, it becomes quite difficult to use them for a heat-radiation substrate material of an IC and the like to be used below 100.degree. C., since it requires insulation of it. In addition, in the case of ceramics having high thermal conductivity like aluminum nitride and silicon carbide, it has been difficult to employ them fog practical use since they are lacking in mechanical strength.